1. Field of the Invention
This invention relates to a process and apparatus for manufacturing optical storage devices such as microfilms, microforms and microfiches, and also larger recording media, such as A4 size sheets, using liquid crystalline materials.
2. Description of Related Art
Liquid crystals, in general, are unique materials which exhibit anisotropy of various physical properties, including refractive index and dielectric susceptibility. These properties allow liquid crystal (LC) materials to be controlled by electric fields and thereby to provide the responses commonly found in liquid crystal displays (LCDs).
Liquid crystal polymer (LCP) materials are a specific class of liquid crystal materials which exhibit these properties and additionally exhibit the potential advantages of conventional polymer materials, such as mechanical integrity and ease of processing. However, they have not been used in LCDs because they have a sluggish response to electric fields due to their high viscosity, even when heated.
It is known that LCP materials can be used as thermo-optic and photo-optic storage media. The application of LCPs in the area of optical recording is of particular interest and these electro-responsive materials further offer the prospect of erasability in recording media. While it is known that certain classes of low molar mass (non-polymeric) LCs, e.g. smectic A cyanobiphenyl mixtures, can also be implemented in erasable optical storage elements, the readily-available glassy phases, the high viscosity and the different handling properties available in the case of LCP materials make these materials particularly attractive. For example, with regard to a film-based product, the low molar mass smectic A LC storage media are useless, in spite of the fact that the technology for thin flexible plastic LCD production is already well known. This is so because of the limited viscosity range available in such media. Hence, when smectic A LC media are confined in flexible LCD formats, such as might be required in a micrographics application, and are then stressed mechanically, the data written thereon is corrupted due to the formation of scattering textures. These scattering textures originate at the source of the deformation and propagate across the recording field. Used in a rigid configuration in which they are contained between glass plates, on the other hand, these materials offer very high performance and have been successfully used in ultrahigh resolution displays and artwork generators.
Unlike conventional LCs, LCP materials are high molecular weight materials which possess many of the properties of the actual plastics substrates which are used in plastic LCD manufacture. These electroactive polymers are thus more compatible with such substrates, particularly when correctly engineered with respect to their phase behaviour. The scope for molecular engineering is a further advantage of LCP materials, this scope being broader than for the low molar mass materials, since polymers can be copolymerised, crosslinked, plasticised and form interpenetrating networks, etc. Furthermore, because of the size of their molecules, LCPs do not possess the powerful solvent characteristics exhibited by conventional LC materials, which present problems when using organic substrates.
Although advantages of LCPs for optical recording applications have been identified, some fundamental problems exist in their use. The production of high optical quality LCP film of the purity which the electronics industry demands is difficult to achieve. The occurrence of pinholes and impurities in LCP films is detrimental to their operation under electric fields, and results in increased power consumption and ultimately in power shortage. For example, a major class of side chain LCP materials often suffers from contamination by colloidal metal particles as a result of the catalytic system used in their synthesis. In the liquid crystal polymer field, therefore, conventional LCD techniques have been used just to prepare samples, by alignment of small areas, for subsequent experimental evaluation. The LCP materials have been melted and capillary filled, possibly under vacuum, for such evaluation. The conventional processing techniques which are convenient in polymer technology and which are solvent-assisted, such as spin and dip coating, have previously not been used in forming samples where subsequent application of electrical fields for accessing has been needed. Solvent-assisted techniques have been used, however, in cases where electrical accessing is not required, for sample preparation of write-once LCP optical disks employing a photooptical recording mechanism, for a thermally-erasable film-based product addressed by thermal print heads for so-called "optical whiteboard" applications, and for a write-once photo-optical recording self-supporting film.
To provide electrical access to an LCP film device it would be necessary to provide two substrates, one on each side of the active LCP layer, which also incorporate the optically transparent electrodes. For use in specific applications, such as in micrographics, for example in forming microfiches, thick devices would not be acceptable, because the storage capacity per unit volume would be degraded as a result of the thickness. Furthermore, the use of thick substrates of, say, 200 .mu.m thickness would make contact duplication processes difficult and degrade the quality of duplicated microforms produced from a master. For other applications, however, thicknesses of up to 400 .mu.m or more might be tolerated.
Irrespective of whether the thickness can be accepted, the problem of initialising the recording condition during manufacture of the film to produce a laser addressable thermo-optic autodeveloping device has not previously been resolved.
Furthermore, it is necessary to differentiate between two types of LCP devices which would be applicable in the micrographics field. A first device is a write-once device, as mentioned above, which is updatable only and requires no electrical access and must be as thin as conventional microfiche products. It is not possible to form a laminate with electrodes on opposite sides, to align the polymer using an electric field, and then to remove one of the electrodes to render the device no longer electrically accessible.
A second device requires a double-sided laminate confining the LCP so that electrical access is possible for implementing an edit function. Again it has not previously been possible to initialise the recording condition.